The MK-21 diving helmet is currently the Navy's primary diving helmet for salvage and deep water diving. It replaces the Mk-12 system introduced in 1980, which in turn, replaced the venerable Mk-5 deep sea hard hat, which had been in service since 1916. Based on the Kirby-Morgan Superlite 17 commercial hard hat design, the Mk-21 is lighter than either the Mk-5 or Mk-12 (weighing approximately 27 pounds) and is neutrally buoyant in water. The helmet shell is made of fiberglass and fitted with brass weights. The regulator is similar to a SCUBA second stage and the helmet is supplied with air through a combination surface supplied air / communications line.